Volume 6 Paper C080
Corrosion Monitoring: from laboratory advances to industrial control.
Roland OLTRA
CNRS / Universite de Bourgogne
Laboratoire de Recherches sur la Réactivité des Solides, UMR CNRS 5613
B.P. 47 870
21078 DIJON cedex
FRANCE
Introduction
1. Monitoring strategies: actual and observed evolutions
2. Type and function of corrosion detectors
3. Some example of existing solutions
3.1. EIS inspection of coating
3.2. ECN
4. Integration of sensors
4.1. sensors based on galvanic current measurement
4.1.1. atmospheric corrosion: galvanic current to detect humidity
threshold
4.1.2. crevice corrosion : galvanic current to detect an aggressive
environment
4.2. sensors based on resistance measurement: integration in microelectronic
packaging
4.3. chemical sensors: an example of local integration of pH sensing
5. Discussion: trends in sensor technology
6. Conclusion
Introduction
High-value structures (buildings, bridges, industrial plant, aircraft, ships,
pipelines...) degrade in service and require regular scheduled maintenance and
inspection which is invariably a significant part of the operating costs.
Corrosion detection and monitoring are potential diagnostic and prognostic means
of preserving materials and structures and reducing life-cycle costs of aircraft,
industrial infrastructures, ships, ground vehicles, pipelines and essentially any high
value long working life metallic structure.
The traditional method of corrosion monitoring requires the use of corrosion
coupons. While this method is effective at characterizing an environment's corrosive
potential, the time required for field exposure and analysis may generate results long
after systems have been damaged by corrosive environment.
A non-exhaustive review of "corrosion monitoring" shows rapidly that
"monitoring" is mainly based on very simplified functions like : imaging, listening the
corrosion. But monitoring and in some cases, field inspection, is based on the "traffic
light" concept.
Energy production (nuclear production, oil and gas industry, chemical plant) are
the mainly concerned industrial domains and at the beginning of this century the
demand is increasing in domains like microelectronics, transportation, storage of
nuclear waste …
On the other hand, electrochemical or chemical analytical techniques development
is based on increasing time and space resolution.
The objective of this paper is to see if the evolution of corrosion monitoring would be
based :
• on the transfer of advances of electrochemical and non-electrochemical
methods from laboratory to service
or
• on additional knowledge upon the relation of the monitored parameters with
the time-evolution of the corrosion processes themselves which could be directly
integrated in the pre-processing of the measured signal taking into account the
recent advances in sensor technology.
In this paper, some examples will be given in order to illustrate how the huge
evolution of sensor technology could be taken into account in evolution of corrosion
monitoring. It will be illustrated how sensors are developed to monitor physical
changes and degradation processes.
Some examples will be discussed to show how monitoring evolution can result on
emerging sensors which can be used to monitor the conditions inside and outside a
structure which lead to corrosion.
1. Monitoring strategies: basic and future possible evolutions
The ability to confidently monitor corrosion, particularly in normally inaccessible
areas would allow more cost-effective inspection schedules and would probably be
part of a larger strategy on corrosion management.
The basic concept of monitoring is the "traffic light" concept based status display
of red, amber and green to indicate the state of the in-service equipment [1]. A red
output would require immediate attention, amber would require attention at the next
scheduled service, green would indicate benign conditions, as illustrated in the
following schematic diagram (Fig.1).
Environmental
Factors
Analysis and
Decision
Making
Signal
Conditioning,
Instrumentation
and Data-logging
Corrosion
‘witness’
devices
Corrosion
detectors
Chemical sensors
Electrochemical
sensors
Non destructive
sensors
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2 a possible corrosion monitoring strategy
Figure 1- Schematic diagram showing
The sensors might fall into three categories :
detection of factors influencing corrosion (of which a few are moisture,
temperature, ion concentration and species, pH and electrochemical
potentials)
sensors that are analogues of the system being monitored (which undergo
corrosion processes themselves)
sensors for directly measuring corrosion of the structure upon which they are
mounted (or incorporated into).
The future evolution, which is not completely dependent upon the corrosion
knowledge, will be for such a sensor system to be successful and there are several
criteria it must meet: they must be cost effective, easy to install, reliable and user
friendly (in terms of extracting the data) as well as being able to withstand the harsh
environment for which they are intended.
For example, one future evolution of in-situ corrosion monitoring would be to
produce an array of inexpensive, possibly multifunctional, corrosion detection devices
incorporated into single packages, deployed separately or in small centrally coupled
arrays.
Micro-engineering techniques could be used to fabricate a thin film sensor array
on a variety of substrates, typically silicon or thin film polymers, incorporating say 6
or 12 different types of sensor on every substrate. The sensor would be expected to be
no bigger than a few mm square. For the most ambitious system onboard real-time
data fusion and analysis would play a large part in the overall system.
2. Detectors : the basic function
The basic functionality on which a detector is based can be classified on various
ways as function of the class of detector.
In case of environmental factors, devices can be classified by the factor being
detected
• temperature
• pH
• Optical pH detector pH sensor
• Humidity / Wetness : measured by Linear Polarization Resistance
Monitor, Linear resistance monitor or Resistance monitor
• Ionic compounds: followed by Ion species detectors, like ion selective
thin film – capacitance change or ISFET (Ion-sensitive Field Effect
Transistor [2]).
In case of corrosion witness devices, devices can be numbered for example by
the mechanism used in the device
• electrochemical potential
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• mass loss : this basic corrosion parameter can be recorded in real time
even at low level using for example Quartz microbalance or Vibrating
Silicon Bridge
• corrosion currents : and especially galvanic current (see section 4.2.)
• Resistance change: with a linear resistance monitor with additional
alloy variants
• EIS and derivatives: using thin film EIS sensor solid state electrode or
conventional set-up (see section 3.1.)
• Electrochemical Noise: an example will presented in section 3.2.
• Capacitance : using for example ion selective thin film to measure
capacitance change
In case of corrosion detectors, some electrochemical principles already
mentioned for corrosion witness devices can be found. On the other hand non
destructive techniques, like acoustic emission, are largely spread.
3. Application at large scale: some example of existing solutions
To be effective, a corrosion monitoring program, must be able to distinguish
between different types of corrosion phenomena, for example, corrosion monitoring
techniques need to be able to differentiate between general and localized (pitting)
corrosion. Pitting results from loss of integrity on the surface of the metal and the
development of local anodes and cathodes on the metal surface that drive the
corrosion process. A problem with conventional techniques is that they only measure
the current associated with the overall (general) corrosion process.
Previously, it has required direct examination of corrosion coupons to derive
information on pitting corrosion tendencies.
In the last 20 years, techniques are available that look at the local fluctuations in
the corrosion signals (electrochemical noise - ECN) in addition to the general
corrosion current. These method provide a quick responding signature of the localized
tendencies well before they are manifested in general thinning or highly destructive
pits.
This is a good example of transfer from laboratory to service: on the basis of
fundamental knowledge [3], a specific way of measurement was directly patented [4]
which generated a lot of service applications and again a lot of basic research.
3.1. EIS inspection of coating
Using a portable cell it is possible to perform some inspections. As shown by
DACCO Sci. [5] it was possible to use the sensor under typical coating conditions on
an aircraft (Fig.2). The measurements verify that EIS measurements can be readily
obtained from different areas of an airplane using the hand-held sensor in a depot. No
special preparation or facilities were needed. Furthermore, the sensor could readily
distinguish between good primer and deteriorated primer, which exhibited a decrease
in the low-frequency dependence of at least two orders of magnitude. Measurements
on other aircraft components showed a similar correlation between the EIS signal and
extent of corrosion or other degradation.
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3.2. Electrochemical Noise (ECN)
Figure 2a- EIS spectra taken from three areas of Figure 2b- Engineer performing measurements
an ECN
aircraftmonitoring has been largely proposedwith
sensor
in hand-held
many field
applications. For
example, it has been reported that ECN monitoring can detect effect of the upgrade in
corrosion inhibitor injection pumps on a platform as shown in Fig.3 [6]. In this
figure we can see directly the processed fluctuations signal which originally consists
of the recording of the current and potential fluctuations measured by a conventional
« 3 electrodes » probe. The signal processing allowing calculating the corrosion rate
from the signal fluctuations will not be discussed in this paper (see for example [7]):
the noise resistance Rn is defined as the ratio of the standard deviations of the voltage
and current fluctuations. In many situations the values of Rn are found to be close to
the polarization resistance Rp of the electrodes so that the corrosion rate can be
deduced by means of the Stern-Geary relationship. Nevertheless, some papers
reported that the noise resistance Rn is simply a number, which may or may not be
equal to the polarization resistance of the electrodes under investigation
As presented in Fig.3 the processed corrosion rate shows that :
• Prior to the upgrade, inhibitor was being injected at up to 40ppm - the
average corrosivity value was about 0.35mm/yr.
• Early on 3 June 1997, work started on the pumps. A large dosage of
corrosion inhibitor was being injected, at >70ppm. As observed by the
electrochemical noise corrosion monitoring, the fluid corrosivity in the
“main oil line” dropped immediately to an average 0.15mm/yr.
• After a few hours, the dosage rate was decreased to approximately 4546ppm. The corrosivity increased to 0.25mm/yr average (but still below the
initial value).
The effectiveness of the pump upgrade can be seen quite clearly, and the corrosion
probe was later employed to also demonstrate the effectiveness of new inhibitor
treatments as they were introduced into operation.
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4. Integration of sensors
In the electrochemical sensors, resistance probes, thin film galvanic devices, and
in the field of chemical sensors, specific ion and pH electrodes, H-probe have
emerged as new in-situ devices for measuring or monitoring corrosion condition.
They can give an appreciation of the process of corrosion, locating the sensors in
the right place and the criticality of corrosion in the location context. It means that the
integration of sensors in the working component appears as an attractive evolution.
In the following examples, some examples of integration will be proposed in
various practical domains.
4.1. sensor based on galvanic current measurement
Galvanic sensors have been proposed to follow environmental conditions which
could lead to various cases of corrosion damages
4.1.1. atmospheric corrosion [8]: galvanic current to detect humidity
threshold
The sensor layout for passively monitoring galvanic corrosion between Al and Cu
is shown in Fig.4 . To enhance the signal to noise ratio, the Al nodes can be tied
together into a single working electrode and the Cu nodes can be tied together into a
second working electrode. Alternatively, electrodes can be individually addressed to
gain spatially resolved information. Figure shows the resulting steady-state galvanic
current measured as a function of RH. Between 50% and 60% RH galvanic corrosion
either ceases or falls below the detection limit of the instrumentation.
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Figure 4a. Alternating Al-Cu foil
arrangement used for passively
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Figure 4b. Galvanic current measured
between Al (anode) and Cu (cathode)
This sensor would be also employed for example in nuclear waste storage to
detect any kind of change in humidity. For example, a set of similar galvanic couple
sensors has been developed in line with the concept put forth by Shinohara et al. [4].
These sensors consist of an interdigitated array of silver that is electrically isolated
from the substrate.
The substrates investigated thus far are carbon steel and Type 304L SS. Through
the use of two substrates a relative corrosivity scale can be developed with the carbon
steel/Ag system being more sensitive to a low overall corrosivity whereas, given the
increased resistance of Type 304L SS to corrosion, the Type 304L SS/Ag system
would respond only in more aggressive environments. The possibility that the Type
304L SS/Ag system may be used to detect the onset of localized corrosion is also
being investigated. The drift is being heated to an air temperature of about 105 °C to
simulate the thermal load that would result from radioactive decay within the waste
containers.
4.1.2. Crevice corrosion : galvanic current to detect an aggressive
environment
Hidden corrosion generated in overlapped structures, a phenomenon having
usually a crevice nature, seems for example effectively monitored by means of thin
film galvanic sensors, capable to detect in-situ the presence of an aggressive microenvironment. This is a case of the hidden corrosion phenomena very frequently
observed on aircraft structures, which develop in overlapped structures or under an
aluminum cover sheet. Moisture trapped inside the natural crevices produces
corrosion in agreement with an aeration deficiency mechanism that makes anodic the
crevice with regards to the external area because of the difficulty in supplying oxygen
.
The amount (concentration) of the aggressive species is then proportional to the
galvanic current flowing on these sensors and stored on a data logger easy to
download usually every 4-6 months. Depending on the expected micro-environment,
the galvanic couple can be suitably modified to optimize its duration.
Thin film galvanic sensors (Fig.5a), developed in the recent past at the U.S. Naval
Air Warfare Centre, have been tested [9] using the concept of electrochemical
dissimilarity in metals. The metallic elements of sensor were micro-dimensional-strips
of about 0.15mm in width and 0.05mm in thickness with several alternating circles of
noble and active metals (e.g., gold and iron, gold and zinc, gold and cadmium or gold
and tin) insulated from each other by the base polymer in 0.15mm or less gaps. The
gap between the electrodes of the sensor were made very close to each other so that a
condensed thin film of moisture formed even at 60% relative humidity could easily bridge
it.
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Depending on the expected micro-environment, the galvanic couple can be suitably
modified to provide a long term life and these devices are capable to detect in-situ the
presence of an aggressive micro-environment.
These devices have shown correlation for environmental corrosivity, paint and
barrier degradation and hidden corrosion detection but they are only single-function
devices reporting on single aspects of the progress of corrosion which is measured
indirectly, principally by detecting moisture.
Figure 5b – Sensor embedded in a lap structure
Galvanic Current / m icroam p
Figure 5aEnvironmental
sensor.
10
Adhesive failure
1
0.1
0
200
400
600
800
1000
Time / Hours
Figure 6- Corrosion sensor output – structure described in Fig. 5b exposed to salt spray
(NaCl5%)
As shown in Fig.6, they are kept isolated until moisture from the environment
bridges the two electrodes: when it occurs the sensors will develop a galvanic current
directly proportional to the corrosivity of the trapped moisture which have penetrated
through the sealant and reached the sensor element. It was reported [9] that after the
exposure the fixture was disassembled and examined. It was observed at the interface
area surrounding the fasteners a breakdown of the sealant was evident.
These sensors are normally supplied with a single data logger for off line
information analysis and are still large, of the order of centimeters in dimension. The
current flown will be then stored on a data logger easy to download usually every
4-6 months.
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4.2. resistance: integration in microelectronic packaging
Corrosion is the most frequent cause failure in assembly and packaging of
microelectronic devices [4]. Online monitoring of corrosion on single chip enables
predicting defects like opens and in this way eliminates a risk of a failure of the whole
system. The idea is based upon the concept: corrosion sensors are placed at the
different locations of the chip as a prevention measure. Corrosion monitoring
performs on-chip test and checks if a corrosion sensor still operates within assigned
conditions.
Two solutions can be considered for implementation :
• The first one assumes external software support when the corrosion
monitoring module only sends information about the advance of corrosion,
which is interpreted by an external device.
• The second solution assumes that the monitoring module includes
supporting modules for storage and comparison purposes so the result of
measurements is in the form of pass/fail. The first solution is more flexible
and easy to monitor.
The corrosion sensor (Fig.7a) is implemented as an integrated part of the chip
where the critical role is its size resulting in a minimal consumption of the silicon
area. Recently there were implemented corrosion sensors based on triple track
structures were developed [2].
The concept of triple track structure is based on resistance measurement a
schematic layout of the sensor structure is depicted in Fig.7b.
During assembly test resistance of the passive electrical structure is measured.
Corrosion changes the area of the material, so it results in resistance change.
Corrosion Sensor
Unpassivated Monitored Resistor
Corrosion Monitoring Module
Passivated
"Golden"
Resistor
Ohm
Comparator
OUT
Resistance
Tuner
-
+
V
Corrosion Register
Figure 7a- Triple Track Corrosion Test
Structure
Run_ Test
Figure 7b- The concept of corrosion
monitoring module
4.3. chemical sensors: an example of local integration of pH sensing
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Chemical sensors, which change color when corrosion is present, have been in use
for a very long time. They are known as fluorescent liquid penetrant or fluorescence
indicators.
A new family of fluorescent dyes allows the detection of rust and surface defects
that are hidden under a coat of paint. This new method eliminates the need to remove
the paint prior to aircraft corrosion inspection.
For example, [10] paint systems containing color-change or fluorescing
compounds were found to be sensitive to underlying corrosion processes by reacting
to the pH increase associated with the cathodic reaction that accompanies corrosion.
The sensitivity of acrylic-based coating systems for detection of cathodic reactions
associated with corrosion was determined by applying constant cathodic current and
measuring the charge at which color change or fluorescence was detected. Visual
observation of coated samples with the unaided eye can detect changes resulting from
a charge corresponding to a hemispherical pit with depth on the order of 10 µm.
The basic idea of this study is to modify paint in order to make it a sensor for
corrosion since paint covers, and thus has access to, the whole surface of an airplane
(Fig.8). The goal is to sense the cathodic reaction that accompanies the oxidative
corrosion reaction. The main cathodic reaction for any form of atmospheric corrosion
is oxygen reduction:
O2 + 2H2O + 4e- → 4OHFor localized corrosion such as pitting, crevice, and exfoliation corrosion, this
cathodic reaction will tend to occur at more-accessible locations than the anodic
reaction, i.e. nearer to the source of oxygen in the air (Fig.7). This reaction will cause
an increase in the local pH at the location where it occurs, so a paint that is sensitive
to pH increases generated by the cathodic reaction will indirectly be sensing corrosion
occurring nearby.
Others have pursued similar approaches. Color-change pH indicators have been
incorporated into organic coatings as a tool for determining the pH gradients
associated with filiform corrosion beads. Fluorescent dyes were applied to
microelectronic test vehicles to detect pH changes associated with corrosion of Al or
Au metallization under an applied bias in a humid environment. Fluorescing and
color-change dyes have also been applied to Al after corrosion in order to identify the
locations of the hydrous aluminum oxide corrosion product [11].
Figure 8. Schematic description of the location of the cathodic areas where a pH change
can be detected (alcalinisation) due to local crevice corrosion
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5. discussion: trends in sensor technology
It must be asked how corrosion monitoring could benefit from the developments
observed in sensor technology which are based on the permanent technical progress
in the fields of sensor structure, sensor technology and signal processing. The main
trends in development of sensor technology go towards miniaturization and an
increasing use of multi-sensor and wireless systems. As schematized in Fig.9, the
heart of a sensor device is the sensor element depending on the measured quantity. In
the pre-processing unit the sensor signal is transformed in an adequate signal using
analog processing technique. By means of analog-digital converters the digital signal
is finally transferred by an adequate interface to be integrable in a control loop
Sensor
measured quantity
Signal pre-processing
Analog signal
Signal processing
Digital signal
Interface
Bus compatible signal
Figure 9- Standard sensor structure
In the case of corrosion monitoring what would be the possible evolution of such a
sensor structure?
In the frame of this presentation, the discussion will be mainly related to the
emerging or not new sensing function (measured quantity) and the possible evolution
in the pre-processing and processing steps.
Signal processing
Signal pre-processing
Sensor
measured
quantity
Self test
Additional knowledge
Figure 10- Sensor structure with self test based on neural network training
• Firstly, electrochemical quantity seems to be restricted to already mentioned in
section 2: it means that no fundamental changes are expected in the future.
Nevertheless, evolution can be planned in the field of environmental analysis
following the large development of chemical sensors (new detectors like
chemical nose) allowing to refine the detection of aggressive environments.
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• Secondly, regarding signal pre-processing, self test can be supported by
studies for example on neural network which could be directly integrated in
the internal memory of the sensor. Sensors are available to detect a wide
range of physical and chemical conditions and in some circumstances this may
be sufficient. However, in order to exploit sensors to predict performance and
extrapolate localized sensor measurements to describe a larger structure there
needs to be an element of intelligent interpretation (Fig.10). For example, in
case of environmental sensing, one approach would be to use a mechanismbased model that calculates a microclimate in zones within the structure and
then would predict the degradation that occurs. This can be done by
developing "training" of the sensor after pre-processing step using for example
neural network approach. Consequently there is a need to promote research in
interpretation of measured data in the frame of real sensing condition to define
the threshold values for the levels of risk.
• Finally, concerning the signal processing, the sensors employed will be of
several different types, some based on standard microfabrication technology
(e.g. electrochemical sensors) and some which will require novel functional
materials. The sensors may act alone if simple detection is all that is needed
but a more ambitious aim will be to allow the sensors to measure local
environmental conditions that are then fed into the predictive model. In this
way, a small number of distributed sensors allied to a model that interprets
their output will be able to calculate the degradation in a large complex
structure. The nature of the substrate itself may also vary from location to
location from being a single silicon backplane to a flexible polyimide thin
film.
At present, each sensor operates as a single unit with a dedicated datalogger.
The stored information is downloaded to a personal digital organizer at
appropriate intervals which could be up to several years. In the future,
wireless micro-sensors could be embedded in these structures to continuously
monitor their "health" from the inside out. For example, chloride threshold
sensors can be embedded in bridge decks to monitor chloride ingress into
bridge-deck concrete important in prioritizing steps to protect the underlying
reinforcing steel bars from corrosion. The device could be embedded when
pouring new concrete, as well. The reader could then drive past the sensor to
obtain its chloride-threshold data. Using a global positioning system (GPS),
the health status of the bridge could be automatically updated in the bridge
database.
6. Conclusion
New trends in corrosion monitoring would be mainly related to the development
of new intelligent corrosivity sensors with remote data transmission through a hand
held transponder as it can be already observed especially in the domain of
transportation.
Some sensors could be sophisticated multifunctional sensors which, in case of
airplanes for example, measure actual corrosion on a model alloy sample, monitor
paint breakdown, measure changes in paint coatings or indicate pre-determined levels
of moisture and ionic species.
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Integration of such sensors in corrosion monitoring will not induce emerging
“measured quantity”, but will require additional knowledge related to the actual
measurement information and the corrosion process evolution itself in order to
integrate self-test or self-calibration in the pre-processing step.
On the other hand, concerning the signal processing, corrosion monitoring must be
microprocessor-based instruments for data gathering and networking with multi-array
of embedded sensors and wireless communication is also an interesting option
applicable to corrosion monitoring.
References
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M. Hebbron , BAE SYSTEMS (Operations) Ltd, private communication
2
P. Bergveld, "Development of an ion-sensitive solid-state device for neurophysiological
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3
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4
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5
Web page http://www.daccosci.com
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F. D. Wall, M. A. Martinez, "Quantifying Atmospheric Corrosion Using Stacked Foil
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10
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11
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